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984                            Appendix V,

Its notch in the bar 49, when arriving near the end of the
measured stroke, and the saddle still proceeding leftward, the
bar 45 is relatively dragged rightward, again causing stud 45-46
to be released, and so to drop the worm out of gear. Bolted to
the faces of the hexagon head are specially shaped tools, while
boring bars may be passed through holes 55 and be held firmly
by bolts 56. The head is rotated into its various positions by
hand, the spring stud 57 deciding the same. The latter is with-
drawn by a leftward turn of the handle 58, which operates a spur
pinion 59 gearing with a rack on the side of stud 57. Fig. 905
shews two sets of gear at 42-43 which are changed as required
by the handle 60. Finally, 61 and 62 are clamps for fixing the
saddle when doing heavy work.

The Chuck is usually of concentric form : that is, the three
jaws are operated simultaneously, but an independent chuck
may be used if desired. Both of these have been described
for other lathes at p. 154. The work is fed through the
hollow mandrel B, and a constant stream of oil is poured upon
it to take away the heat from the cut The oil pump 63 is
rotary and belt-driven, and 64 is a collecting trough for oil and
shavings.

Special Tools and Holders.—As the Hexagon lathe is prin-
cipally used for repetition work, it generally pays to spend some
time designing the cutting tools to suit, and it is impossible to
shew here all that can be done in such a direction. A few of the
more important tool holders made by Messrs. Herbert are, how-
ever, illustrated in Fig. 910, all of which are intended for bolting
to the hexagonal head, A is a ' dead' centre for converting the
hexagon into a poppet head : it is provided with a fine adjusting
screw. B shews a four-flute drill in its holder, and c a floating
reamer, held by bolt d and set up by set screws e as may or may
not be needed. D is a combination boring and facing tool. The
bar is fixed in the holder by the clamp /, and is steadied in
previously chucked and drilled work at ^, the cutter being fixed
at c. The facing cutters a a may be ground to any desired shape
and fixed in heads on the holder, one or more of which, as A, are
adjustable longitudinally. The turning tool holder E is most
interesting, h is the cutting tool, which is ground-to a broad